1. Field of The Invention
The present invention relates to a catalyst for acrolein oxidation, more particularly, to production of the catalyst containing molybdenum, vanadium, and tungsten as essential components.
2. Description of the Related Technology
In producing of acrylic acid, oxidation reactions of acrolein with oxygen molecules in the presence of a catalyst have been widely used. The preparation of the catalyst has been researched to obtain a high yield of the acrylic acid, which are primarily directed to the components of the catalyst and the composition thereof. Also, some methods of producing a carrier-retained catalyst have been provided in the prior art.
Japanese Patent Application Laid-Open Nos. Showa 49-117419, Showa 58-166939, and Showa 64-63543, and European Patent Application Laid-Open No. 293,859/1988 disclose methods for producing a carrier-retained catalyst, in which aqueous solutions of metallic salts of catalyst components are mixed and coprecipitated to produce a suspension of a catalyst. An inert carrier, which has a small surface area and a large aperture ratio, such as round or cylindrical silicon carbide, silica, and silica-alumina, is added to the suspension, and water is evaporated by heating the suspension with agitation to produce a catalyst retained within the structure of the inert carrier.
Further, in U.S. Pat. Nos. 4,157,987, 4,259,211 and 4,892,856; and Korean Patent Application Laid-Open No. 7409/1993, a suspension of a catalyst is heated to evaporate water while being stirred. Anhydrous solid of the catalyst is obtained and is ground to powder, which is coated on an inert carrier, such as alundum, by using a coater.
The inert carriers retain the catalyst in their structures and prevent the release of the catalyst at a time. Accordingly, occurrence of excessive oxidation reactions, which may be caused by supply of abundant catalyst, can be avoided. Also, the inert carrier functions as heat buffer by absorbing the heat generated during the oxidation reaction.
Korean Patent Application Laid-Open No. 7409/1993 and U.S. Pat. No. 4,892,856/1990 disclose that physical properties, such as non-surface area, pore volume, and pore diameter distribution, vary in the catalysts prepared even from the identical component metallic salts and composition thereof. The variance in the physical properties of catalysts results in the variance in the catalytic performance of catalysts, i.e., the acrolein turnover ratio and acrylic acid yield. This means the physical properties and accordingly the catalytic performance of the catalysts change, depending on the preparational manipulations as well as the conditions thereof, which also causes the lack of reproducibility in preparing the catalyst. In addition, these variances of the catalytic performance sometimes exceed those by the changes in the components and the composition of the catalyst.
However, there has not yet been a report, which satisfactorily addresses that the physical properties and accordingly catalytic performance of a catalyst change, depending on the process of producing the catalyst including the preparation of a suspension or powder therefrom.
Meantime, in the preparation of the aqueous solution of metallic salts, an excessive amount of water is required to dissolve some metallic salts having low solubility in water, such as ammonium metavanadate and ammonium paratungstate. The amount of water in the suspension of the catalyst is from about 5 to about 10 times by weight of the salts. The solubility improves when the temperature of water increases, but heating of the aqueous solution of the salts deteriorates the catalytic performance.
The water used to prepare the suspension has to be completely removed to form a powder catalyst. Accordingly, the amount of energy and time required to remove the water has a direct relation to the amount of water used. Further, in the case where an inert carrier is added to the suspension to produce a carrier-retained catalyst, additional time is required to remove the water within the carrier structure.
The present invention provides a method of coating a catalyst to a support. The method comprises dissolving salts of metallic components of a catalyst in liquid to form a suspension of particles of the catalyst; controlling a speed of precipitation of the catalyst particles in the liquid; and coating the catalyst on an inert support by applying the suspension of the catalyst particles to the support. The salts of metallic components comprising molybdenum, tungsten, and vanadium.
The controlling the speed of the precipitation comprises homogenizing the catalyst particles suspended in the liquid so that phase separation between the catalyst particles and the liquid can be substantially slowed down. The catalyst particles are homogenized by at least one selected from the group consisting of ball mill, attrition mill, dynamo mill, homogenizer, and supersonic homogenizer. Most of the catalyst particles are homogenized to a size of less than about 10 microns in diameter. The homogenization of the suspension step is conducted either during or after the preparation of the suspension.
The catalyst is coated by spraying the suspension to the support and simultaneously drying the support. The catalyst particles are coated by one of the group consisting of a rotatory coater, a sugar coater, a centrifugal flow coater, and a pherudizer.
The inventors of the present invention have discovered that properties of a catalyst suspended in the water solution change when the suspension of the catalyst is heated to a high temperature. In addition, the properties of the catalyst change when the catalyst suspension is heated for a long period of time. The property changes cause deterioration of the catalytic activity or performance later. This also leads to a reduction in the reproducibility of the catalyst, and the performance of the powder catalyst or the carrier-retained catalyst becomes hard to control.
However, as noted above in the background of the invention supra, to reduce the heat application to the catalyst suspension, it is required to minimize the amount of water needed to dissolve the metallic salts. In this regard, the inventors also have discovered that some metallic salts dissolve in water better when they dissolve in an aqueous solution mixture of other metallic salts than when they dissolve in water separately. Specifically, molybdate, vanadate and tungstate, which are essential to produce a highly active catalyst, dissolve in water and form a concentrated aqueous solution mixture with higher solubility than each of the separate aqueous solutions thereof.
In accordance with one aspect of the present invention, first metallic salt components of the catalyst, which have low water-solubility, are dissolved together in water to form an aqueous solution of the salts. Each metallic salt dissolves in the aqueous solution more than it dissolves in water alone. The remaining metallic salt components of the catalyst, which have high water-solubility, or the aqueous solution thereof are added to the aqueous solution prepared above to form a catalyst suspension. The amount of water required to prepare the suspension of the catalyst can be drastically reduced, which in turn reduces the time and energy in removing the water to produce a powder catalyst in the following step. Further, since less heat is required to evaporate the water from the suspension, the property changes of the catalyst due to the heat application for a long time, which may cause the deterioration of the catalytic performance, do not appear.
In the preparation of a catalyst suspension, molybdate, vanadate and tungstate are dissolved in water at a temperature of at least about 90xc2x0 C., preferably in boiling water. Once they are completely dissolved, the aqueous solution is cooled to a temperature between about 60xc2x0 C. and about 80xc2x0 C. The remaining salts of metal A and metal B, or an aqueous solution of these metallic salts, are added to produce a catalyst suspension. Here, metal A is at least one element selected from the group consisting of iron, copper, bismuth, chromium, tin, antimony, nickel, cobalt, manganese, cerium and thallium, and metal B is at least one element selected from the group consisting of an alkali metal and an alkali earth metal. The total amount of water with reference to the amount of the metallic salt components of the catalyst by weight is about 0.8 to about 5 times, and preferably about 1 to about 2 times.
The water is evaporated from the suspension by heating and an anhydrous solid of the catalyst remains. The solid is ground and dried to obtain a powder catalyst, which is represented by the Chemical Formula I below. The powder catalyst is finally coated on an inert carrier, such as alundum, to produce a carrier-retained catalyst. Alternatively, an inert carrier, such as round or cylindrical silicon carbide, silica, and silica-alumina, is added to the catalyst suspension, and the water is evaporated by heating the suspension with the inert carrier to produce a catalyst retained within the structure of the inert carrier.
MoaWbVcAdBeOxxe2x80x83xe2x80x83[Chemical Formula 1]
Wherein, a, b, c, d, e and x respectively indicate the atomic ratio for Mo, W, V, A, B and O, in which when a=10, then b=1.5 to 4, c=1 to 5, d=1 to 4, and e=0 to 2 and x is determined according to oxidation states of the other elements.
The performance of the dried catalyst powder is superior to that of the dried catalyst produced by using a large amount of water in the preparation of a catalyst suspension, which is heated at an identical temperature as above and for a long period of time. Ultimately, the performance of the carrier-retained catalysts is also improved.
In accordance with another aspect of the present invention, provided is an alternative method of producing a carrier-retained catalyst. The catalyst suspension is sprayed onto an inert carrier, while it is dried by heated air.
A catalytic suspension is prepared by the above method in accordance with one aspect of the present invention or any other method known in the art. However, when a catalyst suspension is prepared by mixing and stirring two different aqueous solutions, in which one has the metallic components as cation and the other has the metallic components as anion, the catalyst particles promptly settle down. When the agitation is discontinued, the catalyst particles precipitate and form phase separation from water.
When the catalyst suspension is sprayed on the carriers to produce carrier-retained catalysts, it is difficult to obtain homogeneous coating if the catalyst particles settle downward and phase separation occurs. In addition, if the settling speed of the particles is fast, the transfer and spray of the suspension by pumping is not possible. The inhomogeneity of the suspension may also affect the catalytic performance and the reproducibility of the catalyst in producing the carrier-retained catalyst.
According to the present invention, the crystal particles suspended are advantageously splitter or ground into smaller particles to maintain the homogeneous suspension of the catalyst. At least one of ball mill, attrition mill, dynamo mill, homogenizer, or supersonic homogenizer can be advantageously used to split or ground the catalyst particles with our without agitation. Any conventional method for making particle size smaller can also be used.
The catalyst particles are split or ground so that the suspended particles can be transferred and sprayed through a nozzle without being stuck. The size of the particles in the suspension advantageously has the diameter of less than about 10 microns. The particles of these sizes settle slowly and do not cause the phase separation even at slow agitation, which makes the catalyst suspension homogeneous. As a result, the settling speed of the precipitate is controlled so that spray coating of the catalyst suspension can be possible. The suspension, in which the particle sizes get smaller, is sprayed onto inert carriers and simultaneously dried with heated air to produce the carrier-retained catalyst.
The splitting or grinding the particles can be made after the complete production of the catalyst particle suspension from the aqueous solution of the metallic salt components of the catalyst. This operation can also be made during the production of the catalyst particle suspension, and advantageously simultaneously with the production of the catalyst particles.
According to the present invention, the inert carrier can be at least one from the group consisting of alundum, silica-alumina, and silicon carbide. Advantageously, the catalyst can also be coated by a rotatory sugar coater, centrifugal flow coater, or a pherudizer.
According to the present invention, for the metals of molybdenum, vanadium, and tungsten, any salts can be advantageously used. For those of metals A and B, a nitrate, acetate, carbonate, and organate can be used, but chloride or sulfate of them is not preferable.
A gas phase oxidation reaction in the presence of the carrier-retained catalyst produced according to the present invention is carried out in the manner known in the art. For example, 1 to 10 volume % of acrolein, 1 to 15 volume % of oxygen molecules, 5 to 60 volume % of aqueous vapor, and 20 to 80 volume % of inert gas (totally 100%) react in the presence of the catalyst at a temperature of 200 to 350xc2x0 C. between atmospheric pressure and 3 atmospheric pressures, at a space velocity (STP) of 500 to 4,000 hrxe2x88x921.
The present invention will now be further described by the following examples: